In a beacon mode of operation, radars interrogate transponders. The transponders may provide a coded response that is interpreted by the interrogating radar. Thus, in addition to the standard range and directional information that radars receive, a radar in a beacon mode may receive other types of information such as the identification friend or foe (IFF) or surveillance data by decoding the coded response. One class of radar transponders do not repeat received radar pulses and instead simply provide a coded response upon detection of radar interrogation. However, it may be desired that the transponder repeat the interrogating radar pulses. Should there be no coding of the repeated pulses, such a transponder could be designed using a low noise amplifier to amplify the received pulses so that they may be re-transmitted. A transponder configured to simply repeat received radar pulses in this fashion acts a type of “electrical mirror” to the interrogating radar.
The repetition of received radar pulses becomes more complicated if a code is to be imposed upon the repeated pulses. To impose a code, the pulse envelope on the received pulses must be stripped from the carrier frequency. The transponder may then modulate the pulse envelope with the desired code. The modulated pulse envelope is then multiplied with the carrier frequency, amplified, and re-transmitted back to the interrogating radar. But note the problem that arises in a wideband design. Because of the wideband operation, the transponder has no way of knowing the particular carrier frequency an interrogating radar may be using. Thus, the transponder would need some means such as a phase-locked loop to lock onto the carrier frequency before the envelope detection stage. Because of this retention of the carrier signal, the transponder may then multiply it back onto the code-modulated pulse envelope. However, the inclusion of a phase-locked loop increases power demands and manufacturing costs. Furthermore, wideband width application of the transponder demands use of a more elaborate frequency synthesizer system.
Accordingly, there is a need in the art for improved radar transponder architectures for the repetition of coded radar pulses.